This invention relates generally to a method for preparing a hydrolysate of organoalkoxysilanes, in particular a hydrolysate in which the hydrophobic organic and low polar molecules exhibit excellent solubility, as well as the use of this organoalkoxysilane hydrolysate for obtaining transparent films or substrates, including or not organic molecules and/or inorganic particles, and the applications of these substrates and films in the field of optics, in particular ophthalmic optics.
Below in this request, the organoalkoxysilane hydrolysate will be called organo-silicon sol.
Generally, preparing organo-silicon sols is difficult.
The final properties of the sol and consequently of the derived substrates and/or transparent films depend to a large extent on the sol preparation method, even if the final sol drying/condensation step for obtaining the substrate and/or the film also plays a significant role.
Such sols should exhibit stability properties, i.e. after its preparation, the essential characteristics of the sol (condensation rate, proportion of the various hydrolysed and/or precondensed species, viscosity) do not change or very little with time.
Besides, in the optical field, it has also be sought to obtain organo-silicon sols capable of solubilising low polar, hydrophobic organic additives, in particular for obtaining films of a few microns in thickness.
This latter property of the sol must be preserved during the drying step, i.e. when eliminating solubilisation solvents and during final condensation of the species derived from hydrolysis, so that the additive does not precipitate during this step.
Among organic additives that are particularly interesting in the optical field, photochromic compounds can be mentioned.
The document FR-A-2 704 851 describes a method for preparing an organo-silicon sol in which the following operations are conducted: complete hydrolysis of a solution containing one or several organo-alkoxysilanes in an organic solvent or mixtures of organic solvents using an acid aqueous solution with a pH equal to or smaller than 3, elimination of the organic solvent(s) and of the residual alcohols and concentration of the solution by distillation for obtaining a sol.
However, the method of the patent FR-A-2 704 851 leads to sols certain properties of which strongly vary with time, in particular the condensation rate and the composition of the species present in the sol.
Moreover, it is difficult to solubilise in the sols of patent FR-A-2 704 851 low polar, hydrophobic organic additives and in particular photochromic compounds.
The article entitled xe2x80x9cOrganosiloxane Resin with High Silanol Contentxe2x80x9d Furuya et al.xe2x80x94Silicones in Coatings IIxe2x80x94A Technology Forum Exploring the Versatility of Siliconexe2x80x94Mar. 24-26, 1998xe2x80x94Floridaxe2x80x94USAxe2x80x94Conference Papersxe2x80x9d, describes the synthesis of an organosiloxane resin by hydrolysing trialkoxysilanes with acidified water in the absence of an organic solvent. The alcohol produced during hydrolysis is eliminated by heating or under reduced pressure in order to precipitate a viscous product that is a siloxane resin with high silanol content.
Although the method of the article leads to more stable sols, it would be nevertheless desirable to obtain sols with increased stability as well as better solubility of additives such as photochromic compounds.
It has been found according to the invention that by hydrolysing an organo-silicon precursor with large water excess, then by concentrating the hydrolysate and by leaving it until segregation into an aqueous phase and an organo-silicon phase, and by dispersing again the collected organo-silicon phase having a very low water content, and possibly dried, in a hydrophobic solvent, a very stable sol could be obtained, in which it was possible to solubilise additives such as photochromic compounds.
According to the invention, the method for preparing an organo-silicon sol comprises
a) hydrolysis of an initial volume Vsi of a precursor material containing at least an organo-silicon monomer precursor with formula:
R1nSi (OR2)4-nxe2x80x83xe2x80x83(I)
in which
the radicals R1, identical or different, represent an alkyl group, an aryl group, a vinyl group or H,
the radicals R2, identical or different, represent H or an alkyl group, and
n is an integer varying from 1 to 2,
n=2 if R1 represents H, with a water quantity such as             x      ⁢              xe2x80x83            ⁢              H        2            ⁢      O              x      ⁢      Si        ≥  10
xe2x80x83and, with a possible quantity of an organic solvent such that   0  ≤            x      ⁢      Solvent              x      ⁢      Si        ≤  8
xe2x80x83where x H2O, x Si and x Solvent represent, respectively, the number of moles of H2O, Si and Solvent present. and under the condition that when                     x        ⁢                  xe2x80x83                ⁢                  H          2                ⁢        O                    x        ⁢        Si              =    10    ,            x      ⁢      Solvent        =    0    ,
xe2x80x83to obtain a hydrolysate of the precursor material;
b) concentration of the hydrolysate down to a volume substantially equal to the initial volume Vsi;
c) leaving the concentrated hydrolysate until a distinct aqueous phase and a distinct organo-silicon phase are obtained, and
d) separation and collection of the organo-silicon phase.
The recovered organo-silicon phase is preferably subjected to a drying step (e), either (1) by addition of a solvent with a boiling point above 100xc2x0 C. at atmospheric pressure or a solvent forming an azeotrope element with water (for example 2-butanone Teb≈79.6xc2x0 C.) and evaporation of the solvent, or (2) by extraction with a hydrophobic solvent.
Using a solvent with a boiling point greater than 100xc2x0 C. calling for heating at relatively high temperature in order to eliminate the solvent, has the shortcoming of causing the soil (condensation rate) to evolve.
Azeotropic distillation, although resorting to lower temperatures, calls for repeated distillations and the quantity of water remaining in the sol remains relatively important.
It is therefore preferable to dry by extraction with a hydrophobic solvent exhibiting a boiling point equal to or smaller than 80xc2x0 C. Preferably, ethyl acetate or diethyl ether is used.
The recommended drying method is diethyl ether extraction that, however, implies replacing ether with another solvent.
Indeed, diethyl ether is not a solvent appropriate for usage of the sol. The sol is polar and its solubility in diethyl ether does not enable to achieve the volume Vsi by evaporation. Moreover, this volatile solvent does not enable the shaping of materials.
Diethyl ether can be replaced easily with any solvent with higher boiling point and in which organo-silicon species are soluble.
Diethyl ether is therefore evaporated partially under reduced pressure (down to the solubility limit of the sol), the replacement solvent is added in excess (for example 2 Vsi), then the evaporation is carried on until the volume Vsi is obtained.
This latter operation is conducted twice in order to evaporate all the diethyl ether present in the sol.
The solvents used are, for instance, acetone, 2-butanone, tetrahydrofuran.
When the diethyl ether extraction step is used both the obtained silicon organic phase and aqueous phase can be extracted with ether and both ethereal phases are gathered before replacing ether with another solvent.
Hydrolysis water is an aqueous solution with a pH ranging generally between 3 and 10, and preferably acid. The hydrolysis solution can be acidified by an inorganic acid such as HCl, HNO3 or H2SO4 or an organic acid such as acetic acid.
As stated above, the quantity of aqueous solution used for hydrolysis is such that the following ratio             x      ⁢              xe2x80x83            ⁢              H        2            ⁢      O              x      ⁢      Si        ≥  10
preferably   10  ≤            x      ⁢              xe2x80x83            ⁢              H        2            ⁢      O              x      ⁢      Si        ≤  20
The hydrolysis medium may comprise an organic solvent selected preferably among THF (tetrahydrofuran), inferior alcohols such as ethanol or inferior ketones such as acetone.
The hydrolysable precursor material comprises at least one organo-silicon monomer precursor with the following formula:
R1nSi (OR2)4-nxe2x80x83xe2x80x83(I)
where R1, R2 and n are such as defined previously.
R1 represents preferably a methyl, ethyl, phenyl radical or a phenyl radical substituted with preferably non-polar groups (for example alkyl groups such as methyl, ethyl, propyl, or phenyl groups or still a vinyl radical.
R2 represents preferably H or a C1 to C7 alkyl group, for example a methyl, ethyl or propyl radical.
Preferably, n is equal to 2 to 1, and ideally n is equal to 1.
Among the particularly preferred organo-silicon precursors with formula (I), the following can be mentioned: methyltrimethoxysilane (MTMOS), methyltriethoxysilane (MTEOS), ethyltriethoxysilane (ETEOS), dimethyldimethoxysilane (DMDMOS), dimethyldiethoxysilane (DMDEOS), diethoxymethylsilane (HMDEOS), phenyltriethoxysilane (PTEOS) and vinyltriethoxysilane (VTEOS).
Apart from a monomer precursor or a mixture of monomer precursors with formula (I), the precursor material may comprises at least one monomer precursor selected among the epoxytrialcoxysilane monomers. Among these epoxytrialcoxysilane monomers, silanes with epoxy group of the following formula can be mentioned: 
in which:
R is a C1-C6, preferably CH3 or C2H5, alkyl group,
Rxe2x80x2 is a methyl group or a hydrogen atom,
a is an integer from 1 to 6, and
b is equal to 0, 1 or 2.
The preferred epoxysilanes are xcex3-glycidoxypropyltrimethoxysilane or xcex3-glycidoxypropyltriethoxysilane.
The particularly preferred monomer is xcex3-glycidoxypropyltrimethoxysilane (GLYMO).
Generally, the resting time to obtain the phase separation (segregation) may vary from 1 to several days up to several weeks, for example 4 to 6 weeks.
To obtain a phase separation (segregation) during the resting step (c), when the precursor material comprises a monomer precursor of formula (I) and an epoxytrialkoxysilane monomer precursor, the proportion in molar percentage, of epoxytrialkoxysilane monomer precursors in relation to the monomer precursors of formula (I), is generally in the order of 50% or less according to the monomers used. Preferably, the molar proportion of epoxytrialkoxysilane monomer precursors with respect to the monomers of formula (I) will be approx. 25% or less.
The elimination step (b) of water and organic solvents can be conducted by any appropriate means, but preferably by application of a primary vacuum.
According to the application contemplated for the organo-silicon sols according to the invention, additives can be introduced to modify the mechanical (elasticity, rigidity, hardness) or optical (index, colour) properties of the end-product, for example by addition of an additive as an organic solution compatible with the organic medium of the sol, then concentration of the sol.
The additives used can be colouring agents such as laser colouring agents, enzymes, semiconductor or magnetic nanoparticles or still photochromic compounds.
The structure of the sols according to the invention, exhibiting high rate of units T2, proves particularly suited to promote spectrokinetic performances of the organic photochromic compounds.
The photochromic compounds used preferably are spirooxazines, chromens or fulgides.
There can also be incorporated in the organo-silicon phase or sol according to the invention a predetermined quantity of colloidal silica, preferably colloidal silica in an organic solvent whose pH ranges between 3.5 and 6, such as the mixed colloidal silica SiO2/Al2O3 (pH 5) in a quantity representing up to 60% by weight of the sol.
The colloidal silica is generally introduced in the final sol preparation step, after recovery of the organo-silicon phase.
The organo-silicon sols according to the invention are characterised, among other things, by a stable condensation rate (Tc), equal to or greater than 0.65 and the presence of a molar content of silicon units T2 greater than or equal to 50%, and preferably greater than or equal to 60%.
Preferably, the organo-silicon sols according to the invention are deprived of water, as determined by the absence of peaks corresponding to water by RMN 1H.
The invention also relates to organo-silicon sols with the previous features and including at least one solubilised photochromic compound.
It has been determined that the solubility of the additives, in particular of the photochromic compounds, was vastly increased with the organo-silicon sols according to the invention.
Besides, the organo-silicon sols according to the invention are very stable.
The organo-silicon sols according to the invention can then be shaped and condensed into massive materials such as xerogels or into thin films.
The sols according to the invention are particularly suited to the realisation of anti-abrasion hard films or anti-reflection films in the field of ophthalmic optics, and especially for spectacle glasses.
The following examples illustrate the present invention In the examples, unless other stated, all the percentages and parts are expressed in weight.